The present invention pertains to improvements to thermal components of a Stirling cycle heat engine and more particularly to heat transfer surfaces such as the heater head.
Stirling cycle machines, including engines and refrigerators, have a long technological heritage, described in detail in Walker, Stirling Engines, Oxford University Press (1980), incorporated herein by reference. The principle underlying the Stirling cycle engine is the mechanical realization of the Stirling thermodynamic cycle: isovolumetric heating of a gas within a cylinder, isothermal expansion of the gas (during which work is performed by driving a piston), isovolumetric cooling, and isothermal compression.
Additional background regarding aspects of Stirling cycle machines and improvements thereto are discussed in Hargreaves, The Phillips Stirling Engine (Elsevier, Amsterdam, 1991) and in co-pending U.S. patent applications Ser. No. 09/115,383, filed Jul. 14, 1998, and Ser. No. 09/115,381, filed Jul. 14, 1998, which reference and both of which applications are herein incorporated by reference.
The principle of operation of a Stirling engine is readily described with reference to FIGS. 1a-1e, wherein identical numerals are used to identify the same or similar parts. Many mechanical layouts of Stirling cycle machines are known in the art, and the particular Stirling engine designated generally by numeral 10 is shown merely for illustrative purposes. In FIGS. 1a to 1d, piston 12 and a displacer 14 move in phased reciprocating motion within cylinders 16 which, in some embodiments of the Stirling engine, may be a single cylinder. A working fluid contained within cylinders 16 is constrained by seals from escaping around piston 12 and displacer 14. The working fluid is chosen for its thermodynamic properties, as discussed in the description below, and is typically helium at a pressure of several atmospheres. The position of displacer 14 governs whether the working fluid is in contact with hot interface 18 or cold interface 20, corresponding, respectively, to the interfaces at which heat is supplied to and extracted from the working fluid. The supply and extraction of heat is discussed in further detail below. The volume of working fluid governed by the position of the piston 12 is referred to as compression space 22.
During the first phase of the engine cycle, the starting condition of which is depicted in FIG. 1a, piston 12 compresses the fluid in compression space 22. The compression occurs at a substantially constant temperature because heat is extracted from the fluid to the ambient environment. The condition of engine 10 after compression is depicted in FIG. 1b. During the second phase of the cycle, displacer 14 moves in the direction of cold interface 20, with the working fluid displaced from the region of cold interface 20 to the region of hot interface 18. This phase may be referred to as the transfer phase. At the end of the transfer phase, the fluid is at a higher pressure since the working fluid has been heated at constant volume. The increased pressure is depicted symbolically in FIG. 1c by the reading of pressure gauge 24.
During the third phase (the expansion stroke) of the engine cycle, the volume of compression space 22 increases as heat is drawn in from outside engine 10, thereby converting heat to work. In practice, heat is provided to the fluid by means of a heater head 100 (shown in FIG. 2) that is discussed in greater detail in the description below. At the end of the expansion phase, compression space 22 is full of cold fluid, as depicted in FIG. 1d. During the fourth phase of the engine cycle, fluid is transferred from the region of hot interface 18 to the region of cold interface 20 by motion of displacer 14 in the opposing sense. At the end of this second transfer phase, the fluid fills compression space 22 and cold interface 20, as depicted in FIG. 1a, and is ready for a repetition of the compression phase. The Stirling cycle is depicted in a P-V (pressure-volume) diagram as shown in FIG. 1e. 
Additionally, on passing from the region of hot interface 18 to the region of cold interface 20, the fluid may pass through a regenerator 134 (shown in FIG. 2). Regenerator 134 is a matrix of material having a large ratio of surface area to volume which serves to absorb heat from the fluid when it enters hot from the region of hot interface 18 and to heat the fluid when it passes from the region of cold interface 20.
Stirling cycle engines have not generally been used in practical applications due to such practical considerations as efficiency, lifetime, and cost which are addressed by the instant invention.
In accordance with preferred embodiments of the present invention, there is provided a method for fabricating heat transfer protuberances, such as for the heater head or cooler of a thermal cycle engine, wherein the heat transfer protuberances conduct heat between an external fluid and a working gas through a cylindrical wall where the working gas is interior to the wall. The method includes casting of the cylindrical wall and the heat transfer protuberances in a single operation. The casting step may include investment casting, sand casting, or die casting. The method may also include steps of fabricating a plurality of negative molds, each mold being of a group of substantially parallel holes corresponding to the heat transfer protuberances in the fabricated part. The plurality of negative molds is assembled to form a negative form for casting the cylindrical wall and heat transfer protuberances.
In accordance with further embodiments of the invention, a method is provided for fabricating heat transfer pins for conducting heat from an external thermal source through a cylindrical wall where the method has the steps of integrally fabricating at least one backing panel and heat transfer pins having axes normal to the backing panel, and then bonding the at least one backing panel to a structure in thermal contact with the cylindrical wall. The step of integrally fabricating the at least one backing panel may include either casting or injection molding the backing panel. The step of bonding may include mechanically attaching the panel to the heater head, brazing the panel of the array of heat transfer pins to the heater head, or transient liquid-phase bonding of the panel of the array of heat transfer pins to the heater head. In accordance with yet further embodiments of the invention, a method is provided for enhancing efficiency of thermal transfer through a heater head to a working gas in a thermal cycle engine, the heater head having an interior surface. The method includes the step of applying a layer of high-thermal-conductivity metal to the at least one of the interior and exterior surfaces of the heater head.
An alternate embodiment of the invention provides an improvement to a heater head for a thermal cycle heat engine that has a substantially cylindrical wall section. The improvement has a plurality of ribs interior to the wall section for providing enhanced hoop strength. Other improvements to a heater head, in accordance with the invention, include a plurality of passages within the wall that extend parallel to a central longitudinal axis and a substantially helical channel within the cylindrical wall section. An additional improvement includes a plurality of ribs interior to the dome for providing enhanced dome strength. A plurality of flow diverters may also be provided, extending transversely from a hot sleeve disposed internally to, and concentrically with, the cylindrical wall section.
In accordance with a further aspect of the present invention, a heat exchanger is provided for transferring thermal energy from a heated external fluid across a cylindrical wall. The heat exchanger has a set of staggered heat transfer protuberances, each heat transfer protuberance having an axis directed substantially away from the cylindrical wall, and a plurality of dividers disposed substantially along the length of the cylindrical wall, for forcing fluid flow through the staggered heat transfer protuberances.
In accordance with yet a further aspects of the present invention, a heat exchanger is provided for transferring thermal energy from a heated external fluid across a cylindrical wall, where the heat exchanger has a set of heat transfer protuberances with axes directed substantially away from the cylindrical wall, and a backer for guiding the heated external fluid in a flow path characterized by a direction substantially along the length of the cylindrical wall past the set of heat transfer protuberances. A gap between the backer and the cylindrical wall may decrease in the direction of the flow path of the external fluid. In other embodiments of the invention, the heat transfer protuberances have a surface area transverse to the flow path that increases in the direction of the flow path. In other embodiments of the invention, the heat transfer pins may have a population density that increases in the direction of the flow path. In yet other embodiments of the invention, at least one of the height and density of the heat transfer pins may vary with distance in the direction of the flow path.